1. Field of the Invention
The present invention relates to a bull gear of a differential gear assembly (a differential bull gear).
2. Related Art
Conventionally, a differential bull gear is made of high-strength metal such as steel or powder metal sintered in high density because its teeth especially require high strength. However, the bull gear entirely made of such high-strength material is expensive.
Therefore, as disclosed in U.S. Pat. Nos. 6,338,690 and 2,608,261, for example, a well-known conventional economical differential bull gear comprises integrally coupled different outer and inner members, wherein an outer member serving as a ring gear is made of high-strength material, and an inner member, a plate which supports a differential pinion, does not require such high strength and may be made of inexpensive material.
However, in this type of bull gear, a problem arises because the two different members coupled together may axially shift from each other so as to hinder proper movement of the differential gear assembly.
The differential bull gear disclosed in U.S. Pat. No. 6,338,690 avoids the axially positional difference between the inner and outer members by inserting corners of the square thin inner member into corresponding recesses formed in the inner peripheral portion of the outer member serving as a ring gear. However, the thin inner member supporting just a differential pinion is so separated from a pair of axles as to make it difficult to fix the axles in axial location when differential side gears fixed on the respective axles mesh with the differential pinion.
Therefore, a differential housing is divided into halves along the bull gear so that the differential housing halves sandwich the bull gear. Each of the differential housing halves is partly extended to fill gaps between the square inner plate and the ring gear so as to reinforce the bull gear and enhance efficiency of torque transmission between the ring gear and the inner plate. However, each of the differential housing halves having such a complicated shape and requiring high-dimensional accuracy may be expensive, and the differential gear assembly is complicated in its assembly.
In the differential bull gear disclosed in the U.S. Pat. No. 2,608,261, the inner member supporting the differential pinion and supported on axles has a circular contour, when axially viewed, so that the peripheral surface thereof entirely contacts a inner peripheral surface of the outer member serving as a ring gear. Both members are prevented from relative rotation by tightening some screws, and both contact heads of the screws so as to be prevented from axially relative shifting.
However, each of female screws for the screws is divided into halves. The female screw halves are formed on the inner peripheral surface of the outer member and the outer peripheral member of the inner member, respectively. The inner and outer members should be accurately located in their relatively rotational and axial directions so as to form the female screws. Since the half-divided female screws require complicated processing and location, and high accuracy, the resultant bull gear may be expensive. If the bull gear is simplified by decreasing the screws and female screws, efficiency of torque transmission between the inner and outer members is reduced, and the screws are further stressed so as to be damaged, causing rotational and axial moving of the inner and outer members.